1. Field of the Invention
The present invention relates to devices for storage of compressed gases and methods for making or using the same. More particularly, it relates to devices for storage of compressed gases, such as hydrogen, in glass capillaries and methods for making or using the same.
2. Related Art
Hydrogen storage is a key challenge for the development of hydrogen energy applications. Two main objectives motivate improvement over steel cylinders. First, the cost of H2 transport must be reduced. For example, the overall capital and operating cost of tube trailers accounts for a major part of the price of delivered hydrogen. Second, the functional requirements of hydrogen energy systems must be met, such as the gravimetric and volumetric density sufficient to allow H2 fuel cell vehicles to match equivalent gasoline vehicles.
The use of steel cylinders has been the most widely used compressed H2 gas storage technology for many years. It provides gravimetric and volumetric storage density of 1 wt % and 16 g/L, respectively. Alternatives to steel cylinders include liquid H2 tanks, composite compressed H2 gas tanks, adsorbents, metal hydrides, and chemical hydrides.
Liquid H2 tanks are typically used for large hydrogen delivery rates and large distances that justify the high capital cost and energy requirements of H2 liquefiers. While they are an established technology, they do not scale down well. For small tanks, the higher surface/volume ratio makes boil-off a major issue.
Composite compressed H2 gas tanks are typically made of an aluminum or polymer liner with a polymer/carbon fiber overwrapping. They are designated type III and type IV. They provide gravimetric and volumetric storage densities of 5 wt % and 26 g/L, respectively. Composite cylinders made with carbon fibers are significantly more expensive than steel cylinders mostly due to the relatively high cost of carbon fibers. High volume production of carbon fibers is not likely to reduce their high cost because of their prevalent use in aerospace composites.
Chemical hydrides are metal-hydrogen compounds that generate hydrogen at the point of use through an irreversible reaction. The spent reaction product(s) needs to be recycled at a central facility. They can provide very high gravimetric capacity (>10% for sodium borohydride). However, chemical hydrides are relatively expensive. Moreover, the operability of H2 generators and the logistics of recycling reaction product(s) are major drawbacks.
Adsorbents operate by physisorption, with H2 molecules being weakly bound to the surface of micro-pores. Appreciable storage capacity is obtained only at low temperature, near 77K. In an adsorbent-based tank, H2 is stored both as adsorbate and in the gas phase. Volume occupied by the adsorbent's skeleton is not accessible to the gas phase. Above a certain pressure this exclusion by the adsorbent's skeleton becomes too penalizing and it is more efficient to remove the adsorbent.
For several years activated carbon was the best adsorbent for cryosorption. However, its performance has not led to commercialization and the use of a combustible adsorbent (such as activated carbon) at low temperature involves risks linked to the potential accumulation of contaminant oxygen.
In the last few years, higher storage capacity was demonstrated in metal-organic frameworks (MOFs). MOFs consist of periodic arrays of metal centers bound by organic linkers. They have very high porosity and well-defined pore sizes. A major drawback of MOFs is the energy and equipment required to extract heat of desorption generated during filling as well as the relatively low delivery pressure at the point of use. This may be contrasted with liquid H2 which can be pumped to high pressure using low power, vaporized using ambient heat, and injected into a high-pressure tank, therefore paying back for the energy spent in liquefaction.
Metal hydrides are formed by dissociation of molecular hydrogen and dissolution of hydrogen atoms in metals. H2 atoms occupy interstitial sites in the crystal structure of metals, intermetallic compounds, alloys, or metallic composites. Hydride formation is accompanied by the release of heat of absorption (typically 30 to 70 kJ/mol), expansion of the crystal structure (by as much as 30%), and decrepitation/settling effects upon cycling. Thus, thermal effects are very important. Significant amounts of heat must be extracted during filling, and injected for discharge. In storage systems, mechanical effects (decrepitation/settling and expansion) must be managed for example by using a polymer/metal hydride composite.
On the positive side, metal hydrides provide low pressure storage, nearly constant discharge pressure at constant temperature (because of the plateau in the pressure-concentration isotherm), and result in purification of the delivered hydrogen since other gases do not dissolve in the metal hydride as easily as hydrogen. Metal hydrides have a good gravimetric capacity (1-10 wt %) and a very high volumetric density (in some hydrides higher than even that of liquid H2).
On the other hand, high-energy ball mills used to produce high-performance hydrides are expensive and result in a high capital cost per kg of hydrogen stored. Milling very fine and reactive metallic powder subjects the production process to significant risks of violent reaction from exposure to air or water. There is no large scale commercialization of metal hydride tanks.
In conclusion, several barriers remain to the commercial introduction of metal hydrides. For industrial gas operations, they simply remain too expensive.
Glass microspheres have been proposed for a number of years as miniature hydrogen storage vessels. R. Teitel: “Hydrogen Storage in Glass Microspheres”, Rept. BNL 51439, Brookhaven National Laboratories, 1981. Glass microspheres are appealing due to the fact that the failure of one microsphere is expected to have no consequence in terms of safety since the amount of hydrogen released is minimal. Filling and release is accomplished by heating the microspheres from ambient temperature, at which the permeability of hydrogen in the microspheres is minimal, to a temperature in the range 100-400° C., at which the permeability of hydrogen in the microspheres allows them to be filled or released based upon the pressure difference across the microsphere wall. Thus, a heating source is required. A different scheme was recently proposed using the IR sensitivity of doped glass to induce hydrogen release. D. B. Rapp, J. E. Shelby, Journal of Non-Crystalline Solids 349 (2004) 254-259. Nevertheless, the demonstrated capacity of glass microsphere-based systems has so far been considered non-competitive to alternative hydrogen storage techniques. For example, one project demonstrated gravimetric and volumetric capacities of 2.2 wt % and 4 g/L, respectively. J. E. Shelby, Alfred University, DOE HFCIT Annual Merit Review, Apr. 18, 2008. 2.2 wt % is better than Type I cylinders but much lower than Type III and IV cylinders.
RU 2345273 proposed the use of glass capillaries as storage devices. These devices are sealed by a hydrogen permeable material or a low melting point alloy and include a heater for releasing hydrogen. One particular experimental result demonstrates gravimetric capacity of 7.1 to 12 wt % and a fill time of 10 to 30 minutes.
C. En is pursuing the development of a storage system with a gravimetric capacity of 33 wt % and a volumetric density of 45 g/L (An Innovative Technology for Hydrogen Storage in Portable and Mobile Systems, D. Eliezer, Kai Holtappels, Martin Beckmann-Kluge, 18th World Hydrogen Energy Conference 2010). These values are for maximum pressure and do not include any safety margin. The storage system consists of a bundle of glass capillaries (hollow fibers) able to store hydrogen at very high-pressure. The required hollow glass fiber wall thickness for a given pressure is calculated using the known relation:h=r·Pmax/σ,where h is the wall thickness, r the radius, Pmax is the maximum operating pressure times the safety factor, and a the tensile strength of the glass used. When a safety factor of 2 is considered, the foregoing maximum capacity falls to 23 wt % and 44 g/L for extremely thin-wall glass capillaries. To assemble a large number of glass capillaries and store a significant volume of H2, capillaries can be assembled into a fused array as in R. Meyer, 4th ICHS—International Conference on Hydrogen Safety 2011. This method involves high-temperature processing and is inherently expensive.
While the foregoing solutions offer a variety of desirable features, it would be desirable to provide a solution for storage of compressed gases, such as hydrogen, in devices that allow high gravimetric and volumetric densities, are scalable, are not susceptible to boil-off losses, are economical to make and use, do not produce waste products through normal usage, are not susceptible to contamination with oxygen, whose filling and release are not energy-intensive, do not exhibit an undesired degree of decrepitation/settling and expansion, do not incur a significant risk of violent reaction from exposure to air or water during production, which may be filled and released at temperatures less than 100° C., which are easy to fill and release, perform in a robust manner, and may be constructed in a wide variety of simple designs.